Bulk heterojuction (BHJ) polymer solar cells (PSCs) based on conjugated polymers (electron donor (D)) blended with fullerene derivatives (electron acceptors (A)) have attracted great attention due to their potential as renewable energy sources, and the advantages of easy processing and the possibility to fabricate inexpensive, light weight flexible devices. Power conversion efficiency (PCE) of over 8% have been reported for BHJ PSCs under AM1.5G with an illuminating intensity of 100 mW/cm2. However, the major challenge for BHJ PSCs is the demonstration of long-term air stability.
BHJ PSCs are typically fabricated with a transparent conductive anode (e.g. indium tin oxide (ITO)), a low-work-function metal cathode(e.g. Al, Ba/Al and Ca/Ag (Al)), and an active layer (D:A) sandwiched between the anode and the cathode. The cathode is inherently flawed in lower long-term stability. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is often used to smooth the surface of anode, ITO. However, it has been demonstrated that long-term stability is a problem because PEDOT:PSS is hygroscopic and acidic in nature. PEDOT:PSS will etch the ITO, in particular, at ambient condition and accelerate indium diffusing into photoactive layer, then induce a fast degradation of active layer.
Two approaches have been developed to circumvent these problems. One is to develop BHJ PSCs with an inverted device structure. Another is to introduce a stable buffer layer to replace the PEDOT:PSS. Recently, metal oxides, such as molybdenum oxide (MoO3), vanadium pentoxide (V2O5) are thermally deposited onto the ITO anode as a hole transport layer. The elimination of the PEDOT:PSS layer improves the device stability.
Notably, thermal deposition requires the use of complex and expensive equipment, including vacuum chambers and high temperatures, for deposition of the metal oxide on the substrate. High temperature and thus high energy is required to vaporize the solid metal sample. Thus, there is a need in the art for a sol-gel method for thin film deposition of MoO3 in BHJ PSCs. This need extends more generally to sol-gel thin film deposition of hole transport layers of other p-type metal oxides in other semiconductor devices.